The present invention is concerned with the use of hydrogen bis(oxalato)borate of the formula ##STR2## as a protonic acid catalyst in condensation reactions, such as Friedel-Crafts condensations, vinyl ether condensations of acetals with vinyl or propenyl ethers (for the production of intermediates in carotenoid syntheses) and acylations of phenols.
The reaction of 4-hydroxy-2-methyl-naphthalen-1-yl benzoate with isophytol to give 4-hydroxy-2-methyl-3-(3,7,11,15-tetramethyl-hexadec-2-enyl)-naphthalen-1-y l benzoate (dihydro-vitamin K.sub.1 monobenzoate) and of trimethylhydroquinone with isophytol to give d,l-.alpha.-tocopherol (vitamin E) can be mentioned as examples of Friedel-Crafts condensations, the reaction of acetaldehyde dimethyl acetal or (E)-1,1,4,4-tetramethoxy-but-2-ene with methyl propenyl ether or of 13-(2,6,6-trimethyl-cyclohexen-1-yl)-2,7,11-trimethyl-trideca-2,6,8,10,12- pentaen-4-yn-1-al diethyl acetal with ethyl vinyl ether can be mentioned as examples of vinyl ether condensations of acetals with vinyl or propenyl ethers for the production of intermediates in carotenoid syntheses, and the production of d,l-.alpha.-tocopherol acetate from d,l-.alpha.-tocopherol can be mentioned as an example of acylations of phenols.
Various processes for performing Friedel-Crafts condensations have already been described in the literature and are carried out using protonic acids, such as hydrohalic acids, e.g. hydrochloric acid, trichloroacetic acid and the like, Lewis acids, such as aluminium chloride, boron trifluoride, iron chloride, zinc chloride and the like, or mixtures of the two types of acid, such as a mixture of zinc chloride and a strong protonic acid and the like, as catalysts.
All of these previously known processes have serious disadvantages. For example, corrosion problems occur in all processes, and when boron trifluoride is used toxicity problems arise with the boron trifluoride adducts. Also, when iron or zinc is used there is a contamination of the waste water with iron or zinc ions which today is no longer acceptable.
Since vinyl ether condensations are usually carried out with Lewis acids, disadvantages as described above for the Friedel-Crafts condensations with Lewis acids also occur in the case of this type of reaction.